Generally, mineral bodies are blasted with explosives to break up the host rock and enable the broken rock pieces to be removed from the rock body. Blasting with explosives will break up the rock body in a rather crude fashion. The broken rock pieces will generally have a large size distribution with some large pieces of rock and also some small pieces of rock.
While this enables the rock pieces to be physically removed from the host body of rock, further work needs to be done on reducing the size of the rock pieces before they are sent to a beneficiating plant for the recovery of value particles from gangue or waste particles.
The general reduction in the size of particles after blasting is known as comminution and is carried out in crushers and mills. As discussed above, the size of the rock particles needs to be reduced for the subsequent beneficiation process. In particular, the number of large particles from the blasting step should be reduced to as low a level as possible to liberate the valuable mineral in the beneficiation process. It is also desirable to narrow the size distribution of particles that are fed into the beneficiation process, again for liberation of valuable mineral in the subsequent beneficiation process.
The comminution of particles to effect a reduction in particle size is carried out in crushers and mills. Mills may comprise ball or rod mills as well as semi-autogenous grinding (SAG) mills and autogenous grinding (AG) mills.
As a general proposition, mills in the mineral processing industry operate with a low level of efficiency. By this is meant the conversion of input energy, for example electrical energy, into energy that actually breaks the particles is very low. Often the mills are operated crudely based on an operator's understanding of the mill and there is very little science in the operation of the plant. Further the mill settings during operation of the mill are often not adjusted depending on the characteristics of the particles being processed by the mill at any one time.
However, it is well recognised that particle breakage properties for different ore bodies and types of rocks vary greatly. The characterization of particle breakage needs to be better understood and determined. This characterisation of particle breakage can then be used to achieve a greater efficiency in the breakage of particles in a mill.
Clearly, therefore, it would be advantageous if the mill operation could be fine tuned during operation to take into account these differences in particle breakage properties. This would then open up the possibility of more efficient usage of the mills with an improved rate of conversion of input energy to particle breakage within the mill.
The Applicant has developed a prior art test for characterising the breakage of particles that is based on a certain impact energy.
This test is known as the drop weight test and is carried out on laboratory scale equipment in a laboratory to provide some insight into the breakage of particles when subjected to an impact force.
Typically a mine operator sends an ore through to a tester who then conducts the drop weight test on the ore sample for a number of different size fractions. The test results show a size distribution of broken particles for each of the size fractions tested for a certain impact or collision energy.
The test results enable a user to characterise their ores for the design of a mill. This can then be used as an input in the modelling of a mill process, or to assist in optimisation of a given mill or to make changes to the mill settings.
This apparatus, an example of which is illustrated in FIG. 1, comprises a vertical frame 2 extending up from a solid base 3. An impact weight 4 is guided by means of guide rails between an upper position above the base and a lower position in which it collides with a particle 5 that is placed on the base.
In use, a particle to be tested is placed on the base beneath the weight. The weight is lifted up to a certain height and then released allowing it to fall under the influence of gravity. At the bottom of the guide the weight collides with the test particle causing it to break. The broken particles are then recovered and their size distribution can be analysed.
The impact energy that is applied to the particles may be varied. For example, the weight that is placed on the frame may be varied. Further the height from which the weight is dropped can be varied. This enables the breakage properties of a given fraction of particles to be studied for collisions with different input energies or impact forces.
The test described above can be repeated for a number of test particles from the same fraction providing information on how the particle breaks when subjected to that impact energy. It is important that a sufficiently large sample of particles be tested to give statistical validity to the characterisation of particle breakage. Obviously, the greater the number of particles that are tested the better the statistical validity of the results.
Over a number of samples of the same size fraction the results will tend to show how a particle will break for a given impact energy. For example, the particle may break into relatively few particles of about the same size. Alternatively, it may break into many small particles and a few large particles.
A further example apparatus for testing particle breakage properties is shown in FIG. 2.
Basically, the apparatus comprises a frame 6 mounted on a base 7 and extending up therefrom. A rebound pendulum 8 with a block towards its lower end is centrally mounted below the frame in a fixed position and does not move. A rock particle 9 to be tested is mounted in a fixed position on the rebound pendulum.
A swinging impact pendulum is also mounted from the frame and swings like a pendulum below the frame. The impact pendulum is sized and positioned to collide with the rebound pendulum, and specifically the test particle mounted on the rebound pendulum. A collection box for collecting the broken particles from the test particle is positioned below the rebound pendulum.
In use, a rock particle to be tested is positioned on an impact face of the rebound pendulum. An impact pendulum of set weight is lifted up to a set height and then released so that it swings down and then collides with the rebound pendulum. The sample rock on the collision surface of the rebound pendulum is struck by the impact pendulum. This collision causes particle breakage. The broken particles fall into the collection box from where they can be collected and analysed. Typically the particle size distribution of the broken particles is determined using classification screens.
The apparatuses described above with reference to FIGS. 1 and 2 have some limitations.
A first major limitation is that the tests are conducted manually. For each test involving collision with a particle, the particle needs to be placed on the support manually and the weight needs to be lifted and dropped. The broken particles then need to be manually recovered and placed in a sample container for further analysis. The particle size distribution needs to be determined manually using a size classification apparatus.
The process is not automated at all and carrying out tests is very time consuming. Generally, the tests are carried out by a laboratory technician and the labour cost alone of carrying out the tests is substantial.
Further, it will be readily apparent to the skilled addressee that a large number of tests need to be conducted for each size fraction of particle to confer some statistical validity to the results. Generally, 10 to 30 particles of each size fraction need to be subjected to the same test and the results of these tests analysed collectively. However, if only 10 to 30 samples of each particle size are tested the sample size is sub-optimum. This impacts on the statistical validity of the results and the consequent accuracy of the results. From a statistical point of view it would be advantageous if a substantially greater number of particles could be tested for each size fraction, for example testing a sample of 40 to 100 particles per size fraction, or 50 to 70 particles.
A further limitation of the drop weight test described above is that the smallest size of particle that can realistically and practically be tested by the apparatus is 10 mm in diameter. It is very difficult and time consuming to try and mount a particle that is smaller than this on the rebound pendulum. The problem with this is that a sizeable percentage of the particles that are fed into the mill in an operating plant are less than 10 mm in diameter. Thus existing test procedures do not test particles of less than 10 mm and do not provide any insight into their breakage characteristics. By implication, the test results assume that these particles break in the same way as particles that are greater than 10 mm. However, experiments conducted by the Applicant suggest that this assumption is not valid and particles that are less than 10 mm often break differently to the larger particles.
The drop weight tester has a further limitation that will be described below. The Applicant's investigations into the modes of particle breakage within a mill show that there are two types of breakage within a mill. Firstly, there are high energy impacts. Secondly, there is breakage due to repeated small energy impacts. Recent research on the impact energy distribution pattern in an autogenous mill operation has shown that small energy impacts take place at a much higher frequency than high energy impacts. Accordingly, it would be extremely beneficial if a particle breakage tester was capable of characterising particle breakage due to repeated small energy impacts.
When the drop weight tester is used to test particles using impacts at very low specific energy levels, some particles will require as high as 100 repeated hits before they eventually break. This procedure is very time-consuming and labour-extensive to quantify with the drop weight tester. As a compromise, a reduced number of particles could conceivably be used for the incremental breakage test. However, the reduced number of test particles will affect the statistical validity of the test results.
Clearly, therefore, it would be advantageous if an apparatus for testing the breakage characteristics of a particulate material could be devised that ameliorated at least some of these shortcomings.